The present invention relates to electrophotosensitive members, particularly amorphous silicon:germanium photosensitive members.
Amorphous silicon:germanium (hereinafter referred to as a-Si:Ge), because of its high sensitivity toward long wavelength light, is expected for the future as a photosensitive member for printers using semi-conductor laser. Also, since its sensitivity toward short-wave light is not damaged, it can be applied to plain paper copiers (hereinafter referred to as PPC) by regulating the emission spectrum of exposure lamps. Also, a-Si:Ge has excellent feature that, because of its layer well absorbing long wavelength light, there is little disturbance of images by interference of light frequently encountered in the conventional amorphous silicon (a-Si) photosensitive members. Because of these features, many studies for applying a-Si:Ge to photosensitive members are being made.
For example, there are disclosed a technique to use a-Si:Ge over the whole region of photosensitive layer [Japanese Patent Application Kokai (Laid-open) No. 171038/1983]; a technique to apply a-Si:Ge directly on a conductive base of photosensitive member (U.S. Pat. No. 4,490,450); and a technique to apply a-Si:Ge to a layer in direct contact with the surface layer and/or substrate of photosensitive member (ibid., No. 150753/1981). But, either of these techniques is different from the present invention in a position to apply a-Si:Ge. For example, said patent application No. 171038/1983 includes the formation of a-Si:Ge layer over the whole region of photosensitive layer, but a-Si:Ge has its own defect that it is small in .mu..tau. (carrier range) and low in carrier-carrying efficiency. When a-Si:Ge is therefore applied over the whole region of photosensitive layer, generated carriers are trapped by the a-Si:Ge layer to cause not only reduction of sensitivity, but also generation of light fatigue and residual potential.
Also, as described in said U.S. Pat. No. 4,490,450 and in Japanese Patent Application Kokai (Laid-open) No. 150753/1981, when the a-Si:Ge layer has been applied as the base of photosensitive layer, because of a-Si:Ge being easy to generate thermally excited carriers, injection of carriers at the base becomes easy to cause reduction of charging capability. Besides, when the thickness of the a-Si:Ge layer is made large in order to eliminate interference patterns generated in printers using semiconductor laser ray or long-wave coherent light as a light source, carriers present in the vicinity of the base are trapped by the a-Si:Ge layer to cause reduction in sensitivity and generation of light fatigue and residual potential.
Further, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 150753/1981, when the a-Si:Ge layer has been applied to the outermost surface of photosensitive members, carriers excited by short-wave light cannot migrate to move out of the layer to fail to contribute to sensitivity. While, when the thickness of the a-Si:Ge layer is made large in order to inhibit interference of light, carriers are trapped in the layer. Also, a-Si:Ge generates a large number of thermally excited carriers to cause injection of charges from the surface and this obviously lowers the charging capability.
For this reason, the foregoing conventional techniques do not make the best use of the excellent characteristics of a-Si:Ge.
On the other hand, Japanese Patent Application Kokai (Laid-open) No. 154850/1983 discloses an example of providing triple layers of a-Si:Ge to form the photosensitive member which has a photosensitivity extending to the long wavelength region. But the object of this photosensitive member is to control specific resistance and conductivity. This patent application does not refer at all to selection of the position of a-Si:Ge for solving the problems encountered in using a-Si:Ge, i.e. a reduction in carrier-carrying efficiency accompanied by reduction of sensitivity and generation of light fatigue and residual potential.